The present invention relates to organic light emitting (EL) devices and a method for manufacturing the EL devices, and in particular, to a buffer structure in an organic EL device of a top emission system, and a method for fabricating the structure.
An organic electroluminescence device (hereinafter referred to as an organic EL device) comprising a laminated structure of organic thin films is known as an example of light emission devices applied to display apparatuses. Since C. W. Tang of Eastman Kodak Company disclosed a device with a double layer structure performing high efficient light emission in 1987 (Appl. Phys. Lett. vol. 51, p. 913 (1987)), numerous studies have been made to achieve practical application of organic EL devices.
In recent years there has been extensive development in the field of organic EL displays, particularly in the area of active-matrix drive systems for displays. The display in an active-matrix drive system is constructed with light sources of a plurality of organic EL devices formed on a substrate having switching elements of thin film transistors (TFTs). Since the variation in characteristics of the TFT or the organic EL device is great, the conventional display in the active-matrix drive system needs various driving circuits to compensate for the variations. However, complicated driving circuits increases the number of TFTs required to drive one pixel.
A majority of organic EL devices used for displays have a bottom emission type structure in which light is emitted through a glass substrate. FIG. 1(a) is a schematic cross sectional view of a bottom emission type organic EL device. When a bottom emission type organic EL devices is used in a display of the active-matrix drive system, an increase in the number of TFTs causes a decrease in light emission area in the bottom electrode. For this reason, a top emission type structure, in which light is emitted through a top electrode, is a more useful structure than the bottom emission type organic EL device (FIG. 1(a)) for the display on the active-matrix drive system. This type of device also is being developed. FIG. 1(b) is a schematic cross-sectional view of a top emission type organic EL device.
A top electrode of a top emission type organic EL device must be sufficiently transparent. Consequently, the top electrode is generally made of a transparent conductive film made of a substance that has a high transmission rate for visible light and a high electrical conductivity. The transparent conductive films include metallic thin films with a thickness of 5 nm or less of Au, Ag, Cu, Pt, and Pd, oxide semiconductor thin films of SnO2, TiO2, CdO, In2O3, and ZnO, and oxide semiconductor thin films of complex systems of these materials including ITO (indium-tin oxide) and IZO (indium-zinc oxide). Because the metallic thin films exhibit high light absorption and low stability due to small hardness, an oxide semiconductor thin film is primarily used for the transparent conductive film. Transparent conductive films made of ITO or IZO are used for electrodes in wide application areas of TVs, transparent heaters, and liquid crystal display devices.
Although the organic semiconductor thin film of ITO or IZO can be used for a top electrode in a top emission type organic EL device, there remain problems that have to be solved.
The first is to improve low efficiency in electron injection when a top electrode (cathode) of ITO or IZO is used. The electron injection efficiency of a top emission type organic EL device having a cathode of a transparent conductive film of IZO is lower than the electron injection efficiency of a bottom emission type organic EL device having a cathode of a metallic electrode of Al or Ag. This is caused by a difference in material properties as shown in Table 1. IZO exhibits a larger work function than Ag and Al, and a much lower carrier density. Therefore technology is desired to obtain a top emission type organic EL device that has a high electron injection efficiency. This allows enhanced carrier density of the cathode and achieves a work function that is lower than that of a bottom electrode (anode) and matches the numerical value of the work function of a cathode in a bottom emission type organic EL device.
TABLE 1Properties of materials for a top electrode.work functioncarrier densitymaterial(eV)(cm−3)IZO4.801021Ag4.421023Al4.181023
A second problem of top emission type organic EL devices that must be addressed is damage to the organic EL layer during deposition of the top electrode. Sputtering is an efficient and simple method for depositing a thin film, and therefore is often applied to deposition of a transparent conductive film of IZO, for example. However, the energy in the deposition of a film is 300 to 400 eV for the sputtering method, which is much larger than the value of about 0.1 eV for an evaporation method and the value of 20 to 30 eV for an ion plating method. Consequently, when sputtering is used to form the top electrode the organic EL layer underlying the top electrode is liable to be damaged by collisions of high energy particles generated in the sputtering process. The high energy particles can include neutral atoms and negative ions from the target substance, neutral atoms and positive ions from the gas component, and electrons. An organic EL layer damaged in the deposition process often results in deterioration of device performance, including short-circuits, leakage, or lowered luminance efficiency. Accordingly, a technology is desired that reduces the damage on the organic EL layer, which is anticipated when a high energy deposition process such as sputtering is applied to form the top electrode.
The third problem of top emission type organic EL devices is the control of deterioration in the organic EL layer by oxygen. An electron injection layer is generally provided under the cathode to enhance electron injection efficiency of an organic EL device. However, if oxygen is present in the system for depositing the top electrode (cathode), the electron injection layer and the electron transport layer are apt to be oxidized, which may deteriorate performance of the organic EL layer. When a transparent conductive oxide is used as the electrode material, the adverse effect of oxygen cannot be ignored. If the organic EL layer is oxidized with oxygen in the system, the electron injection layer is oxidized, and this changes the original material properties, such that the device may not preserve design performance. When sputtering is used to deposit the top electrode, the organic EL layer also may be damaged by plasma of oxygen in the system originated from oxides or introduced gas. An oxygen plasma inflicts more severe damage on the organic EL layer than an inert gas plasma such as argon, and is apt to substantially lower luminance. Accordingly, a method is desired that controls degradation of the organic EL layer caused by the oxygen existing in the system at the time of deposition process.
In light of these problems, organic EL devices are being studied that have a buffer structure between the organic EL layer and the top electrode (cathode) in order to enhance the electron injection efficiency and to reduce the damage on the organic EL layer in the process for forming the top electrode.
For example, Japanese Unexamined Patent Application Publication No. H10-162959 discloses a top electrode (cathode) that comprises an electron injective metal and an amorphous transparent conductive layer in order to obtain a cathode exhibiting low electrical resistance and enough transparency. According to this document, an extremely thin metal film is provided on the organic EL layer. Such a metal film is, however, too thin to act as a buffer to mitigate the impact of sputtering, although it may improve the electron injection efficiency. Even if the thickness of the metal film is increased to improve resistance to impact of sputtering, desired luminance characteristics are very hard to obtain because the thickness of the metal film and the transmission rate are in a trade-off relationship.
Japanese Unexamined Patent Application Publication No. 2000-58265 discloses providing a buffer layer of phthalocyanine on an organic EL layer, i.e., on an electron transport layer, and diffusing an element such as Ce, Li, Ca, or Mg as a dopant, in order to reduce damage to the organic EL layer due to sputtering and to enhance electron injection efficiency. Although electron injection performance can be improved by means of the buffer layer disclosed in this document, phthalocyanine itself exhibits weak resistance to the impact of sputtering. Thus, the film thickness of from 5 to 100 nm of the buffer layer is insufficient to effectively mitigate impact on the organic EL layer by sputtering.
Japanese Unexamined Patent Application Publication No. 2002-75658 discloses providing a first buffer layer of alkali halide and second buffer layer of phthalocyanine on an organic EL layer, in order to reduce damage to the organic EL layer due to sputtering. Since the alkali halide used to form the first buffer layer is an insulator, film thickness of the first buffer layer is limited to less than 3 nm. On the other hand, the phthalocyanine used to form the second buffer layer is weak against impact of sputtering. This document thus discloses increasing the thickness of the second buffer layer of phthalocyanine to as thick as 200 nm. However, a thicker buffer layer lowers electrical conductivity and optical transmissivity.
Japanese Unexamined Patent Application Publication No. 2002-260862 discloses providing a first buffer layer of alkali halide and second buffer layer of a metal with low work function on an organic EL layer, in order to reduce damage to the organic EL layer due to sputtering. According to this document, the buffer layer with this structure enhances electron injection efficiency, reduces damage due to sputtering, and enhances optical transmissivity. Total thickness of the buffer layers is stated to be preferably less than 5 nm. The cathode (top electrode) disclosed in an embodiment in this document is a metal electrode such as silver or aluminum, which is not suited for application to a top emission type organic EL device. If the top electrode (cathode) is formed of a transparent conductive film of, for example, IZO by sputtering, damage on the organic EL layer is more severe than in the case of a top electrode of a metallic electrode. Consequently, the total thickness of the buffer layers of about 5 nm is insufficient to protect against the impact of sputtering, although improvement may be possible in electron injection efficiency and in optical transmission rate. Even if the thickness of the buffer layers is increased, desirable organic EL characteristics are difficult to obtain because the resistance to sputtering impact and optical transmissivity are in a trade-off relationship.
Various studies of buffer structure have been tried in order to improve electron injection efficiency or to reduce damage to the organic EL layer due to sputtering, as described above. However, the buffer structures studied so far have not allowed simultaneous improvement in electron injection efficiency and reduction of damage to the organic EL layer due to sputtering, while preserving sufficient optical transmissivity and electrical conductivity. Therefore, improvement is still demanded. Particularly in a top emission type organic EL device having a top electrode (cathode) of a transparent conductive film such as IZO, degradation of an organic EL layer due to oxygen cannot be ignored. Thus, a buffer structure is desired that protects the organic EL layer against the oxygen.